+
+/*
+ * rtp_ext_seq:
+ * @extseq: (inout): a previous extended seqs
+ * @seq: a new seq
+ *
+ * Update the @extseq field with the extended seq of @seq
+ * For the first call of the method, @extseq should point to a location
+ * with a value of -1.
+ *
+ * This function is able to handle both forward and backward seqs taking
+ * into account:
+ * - seq wraparound making sure that the returned value is properly increased.
+ * - seq unwraparound making sure that the returned value is properly decreased.
+ *
+ * Returns: The extended seq of @seq or 0 if the result can't go anywhere backwards.
+ *
+ * NOTE: This is a calque of gst_rtp_buffer_ext_timestamp() but with
+ * s/32/16/ and s/64/32/ and s/0xffffffff/0xffff/ and s/timestamp/seqnum/.
+ */
+static guint32
+rtp_ext_seq (guint32 * extseqnum, guint16 seqnum)
+{
+ guint32 result, ext;
+
+ g_return_val_if_fail (extseqnum != NULL, -1);
+
+ ext = *extseqnum;
+
+ if (ext == -1) {
+ result = seqnum;
+ } else {
+ /* pick wraparound counter from previous seqnum and add to new seqnum */
+ result = seqnum + (ext & ~(0xffff));
+
+ /* check for seqnum wraparound */
+ if (result < ext) {
+ guint32 diff = ext - result;
+
+ if (diff > G_MAXINT16) {
+ /* seqnum went backwards more than allowed, we wrap around and get
+ * updated extended seqnum. */
+ result += (1 << 16);
+ }
+ } else {
+ guint32 diff = result - ext;
+
+ if (diff > G_MAXINT16) {
+ if (result < (1 << 16)) {
+ GST_WARNING
+ ("Cannot unwrap, any wrapping took place yet. Returning 0 without updating extended seqnum.");
+ return 0;
+ } else {
+ /* seqnum went forwards more than allowed, we unwrap around and get
+ * updated extended seqnum. */
+ result -= (1 << 16);
+ /* We don't want the extended seqnum storage to go back, ever */
+ return result;
+ }
+ }
+ }
+ }
+
+ *extseqnum = result;
+
+ return result;
+}
+
+